Heat treatment or conditioning is used in many manufacturing processes. In manufacturing certain types of printing plates, for example, it is desirable to subject the printing plates at certain stage or stages of production to elevated temperatures for a sustained period. As an example, as disclosed in the U.S. Pat. No. 6,461,795 (to McCullough et al.; the “'795 patent”), positive-working, heat sensitive, lithographic printing plates have been made by coating lithographic substrates with a phenolic resin composition and, shortly thereafter, heating the plates at 40–90° C. for at least four hours. The heat treatment has been found to improve the exposure processes later. In particular, the heat treatment reduces the variability of photosensitivity of the coated compositions over time.
One practical method for heat treatment that is carried out over long periods of time on multiple printing plates includes arranging the printing plates into a stack and placing the stack in a heating zone of such apparatus as an oven. For example, the '795 patent discloses wrapping multiple printing plates interleaved with paper in a packet for heat treatment. As another example, U.S. Pat. No. 6,596,457 (to Hidaka et al.; the “'457 patent”) discloses stacking hundreds of printing plates for heat treatment. To assure productivity, it is desirable to stack a large number of printing plates for heat treatment at one time. However, due to limited thermal conductivity, especially when printing plates are interleaved with a material, such as paper, that is more insulating than the printing plates themselves, the temperatures near the middle of the stack rise more slowly than near either top of bottom end of the stack. This difference in heating characteristics between different regions has at least two detrimental effects. First, the interior of the stack experiences a longer heating up and cooling down periods than the surface regions of the stack. Thus, to ensure adequate heat treatment of all plates, longer heat treatment cycles must be used. Second, the interior of the stack is heated at the set temperature for a shorter period of time, if at all, than the surface regions during a heat treatment cycle. Thus, the interior of the stack has in essence a different heat treatment cycle than the surface regions of the stack. This difference may result in inconsistencies in printing plate quality or longer heat treatment time needed to ensure adequate heat treatment of the printing plates in the interior of the stack. While removing the interleaved sheets of paper may reduce this difference in heat treatment, it is often impractical because of the need for protective sheets for the printing plates.
The invention disclosed herein is aimed at providing a method and apparatus for more rapid and uniform heating and cooling throughout a stack of printing plates, substantially without many of the drawbacks of the conventional approaches.